Something to bear in mind is that regex is actually a declarative programming language like prolog : your regex is a set of rules which the regex interpreter tries to match against a string. During this matching, the interpreter will assume certain things, and continue assuming them until it comes up against a failure to match, which then causes it to backtrack. Regex assumes "greedy matching" unless explicitly told not to, which can cause a lot of backtracking. A general rule of thumb is that the more backtracking, the slower the matching process.

It is therefore vital, if you are trying to optimise your program to run quickly (and if you can't do without regex), to optimise your regexes to match quickly.

I recommend the use of a tool such as "The Regex Coach" to debug your regex strings.

Note that this setting wasn't present under previous PHP releases and the behaviour (or limit) under those releases was, in practise, higher so all these PCRE functions were able to "capture" longer strings.

With the arrival of the setting, defaulting to 100000 (less than 100K), you won't be able to match/capture strings over that size using, for example "ungreedy" modifiers.

So, in a lot of situations, you'll need to raise that (very small IMO) limit.

The worst part is that PHP simply won't match/capture those strings over pcre.backtrack_limit and will it be 100% silent about that (I think that throwing some NOTICE/WARNING if raised could help a lot to developers).

There is a lot of people suffering this changed behaviour from I've read on forums, bugs and so on).